To change the details on your entry, make sure you're logged into Race Space, then go to the ‘My Entries’ tab. Select 'Entry Actions' on the entry you're trying to update, and then ‘Edit Entry’ from the slider.
From here most fields are editable, except for those that might be viewed as transferring a ticket to someone else, namely 'First name', 'Last name' and 'Email'.
If you wish to Transfer your entry to someone else, see here. If you need to change any of the locked fields, you’ll need to get in contact directly with the event organiser who can change these on your behalf.